1. Field of the Invention
This invention relates to methods of processing a substrate, and more specifically, to a method and apparatus for the delivery of chemical solutions during the cleaning process of, for example, semiconductor substrates.
2. Background Information
In the manufacture of semiconductor devices, the surface of semiconductor substrates must be cleaned of substrate contaminants. If not removed, substrate contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.
A scrubber that scrubs a substrate on either one or both sides may be used to remove substrate contaminants. The type of cleaning solution (solution) used in the scrubber may depend upon the type of contaminants to be removed, the particular type of substrate to be scrubbed, and/or the particular manufacturer's preferred method for cleaning. For example, some manufacturers require a low level of contamination and may use a chemical solution for scrubbing, while other manufacturers have a higher contamination tolerance (i.e. less contamination need be removed) and use water for scrubbing.
As the type of solution depends upon manufacturing requirements, similarly, the method or apparatus used to deliver that solution may depend upon the manufacturer's requirements and the type of solution being used. Submersing the substrate in the solution, spraying the solution on the substrate, and dripping the solution on the substrate or the brush are examples of methods used to deliver the solution for scrubbing. The drip delivery system is described in U.S. Pat. No. 5,723,019 titled "Drip Chemical Delivery Method and Apparatus," issued on Mar. 3, 1998, and assigned to the assignee herein. Each of these methods have their advantages and disadvantages.
Submersing the substrate in the solution requires large volumes of chemical solutions. Some of the solutions, for example, NH.sub.4 OH, can be expensive and toxic to use. Thus, reducing the volume of solution used is desired.
Spraying the substrate also uses large volumes of solutions. Another disadvantage to spraying is that there is very little control over the chemical composition at the substrate surface. For example, some systems and process may use relatively quick bursts of high pH solutions, such that the pH profile of the surfaces may change rapidly and may not be easily controlled. If the pH profile of the surface is not controlled damage may result to the substrate.
Dripping the solution on the substrate or brush uses smaller volumes of the solution, but may result in nonuniform delivery of the solution. Thus, only the areas of the substrate where the solution was dripped may be cleaned. Also, dripping the solution on the substrate may damage the substrate, depending upon the reactivity of the solution. Some solutions may react as soon as they hit the substrate surface and may cause "grooves" or "holes" to be formed where the solution is dripped onto the substrate. Other solutions, for example NH.sub.4 OH, do not react as quickly and do not damage the substrate.
It has also recently become possible to perform chemical etch processes at the same time that the semiconductor substrate is being cleaned or scrubbed. One such process is described in U.S. Pat. No. 5,806,126 titled "Apparatus for a Brush Assembly," issued on Sep. 15, 1998, and assigned to the assignee herein. The process described in U.S. Pat. No. 5,806,126 ('126 Patent) is a combined ammonium hydroxide (NH.sub.4 OH) and hydrofluoric acid (HF) cleaning and etch process. A substrate is first scrubbed in a first brush station using a solution of NH.sub.4 OH to remove particulate contaminants and then the substrate is transported into a second brush station for an etch/cleaning process using HF.
Using the method described in the '126 Patent, particulate contamination is removed in the first brush station using NH.sub.4 OH. NH.sub.4 OH is used to change the zeta potential between the surface of the substrate and the contaminants. The zeta potential is related to the surface energy or "charge" at the surface of the substrate and contaminants. NH.sub.4 OH changes the zeta potential such that the contaminants and substrate surface have potentials which are of like charges. As is well known in science, like charges repel like charges. Thus the substrate surface and contaminants repel one another, thereby removing the contaminants from the substrate surface.
NH.sub.4 OH is applied to the substrate using the delivery system illustrated in FIG. 1. This delivery system is referred to in the '126 Patent as a high concentration delivery system. The high concentration delivery system has two delivery tubes: one for delivery of the NH.sub.4 OH solution and another for delivery of deionized water (DI water). The two delivery tubes are operated such that the deionized water remains turned on and the chemical solution is turned on and off, depending upon when the user wants the chemical solution applied to the substrates. In other words, when NH.sub.4 OH is being used (i.e., turned on) the DI water is turned off and when NH.sub.4 OH is not being used (i.e., turned off) the DI water remains turned on.
When a substrate is transported into the first brush station for cleaning with the NH.sub.4 OH solution, the DI water which has been running to keep the brush moist remains on and the NH.sub.4 OH solution is also turned on. The substrate is scrubbed for a period of time (which is selected by the user) and then the NH.sub.4 OH solution is turned off and the DI water remains on. The DI water remains on after scrubbing with the NH.sub.4 OH solution in order to rinse the substrate (and the brush) to remove any extraneous NH.sub.4 OH that may be on the substrate before the substrate is transported into the next brush station. Such a process creates a chemical profile as illustrated in FIG. 2a.
As illustrated in FIG. 2a, the chemical profile on the substrate is uneven and takes a considerable amount of time before the desired amount of NH.sub.4 OH is delivered to the substrate. At time 210, the substrate has been placed into the first brush station, the NH.sub.4 OH solution is turned on, while the DI water remains on to prevent a high concentration of NH.sub.4 OH solution from being delivered directly to the substrate. The cleaning process with NH.sub.4 OH is performed from time 210 to 220. At time 220, the cleaning process ends and the rinse process begins by turning off the NH.sub.4 OH solution and leaving the DI water turned on.
FIG. 2a also illustrates the amount of time necessary to run such a process. The NH.sub.4 OH cleaning process is long because it takes a significant amount of time to reach the desired level of NH.sub.4 OH (ramp up) and it also takes a significant amount of time to rinse the substrate before the substrate can be transported to the next brush station for the next process (i.e., the HF etch/cleaning process). It should be noted that in order to increase the cleaning of the substrate the only way to do so is to increase the length of time that the substrate is being scrubbed in the first brush station. Thus, the time for cleaning (i.e., 210-220) may be longer than illustrated in FIG. 2a. It should be noted that increasing the concentration of the NH.sub.4 OH solution does not necessarily increase the cleaning. Additionally, if the concentration of the NH.sub.4 OH solution is increased such an increase may affect the pH profile on the surface of the substrate and could potentially damage the substrate.
It should also be noted that in order to reach the desired level of NH.sub.4 OH, the ramp up sequence requires large amounts of the NH.sub.4 OH solution. Using large amounts of chemicals can become costly because some chemicals used in semiconductor substrate cleaning may be more expensive than others.
After the substrate has been rinsed the substrate is transferred at time 230 to the second brush station for the HF etch/cleaning process. Using the method described in the '126 Patent, HF is used to remove a portion of the substrate surface. Because HF is such a highly reactive chemical it is desirable to use HF in lower concentrations in order to control the etch process.
HF is applied to the substrate using the delivery system illustrated in FIG. 3. This delivery system is referred to in the '126 Patent as a low concentration delivery system. The low concentration delivery system has two delivery tubes: one for delivery of the premixed HF solution and another for delivery of de-ionized water (DI water). The two delivery tubes are operated at different times and when one of the delivery tubes is turned on the other delivery tube is turned off. In other words, when premixed HF solution (HF solution) is being used (i.e., turned on) the DI water is turned off and vice versa.
When a substrate is transported into the second brush station for etching with the HF solution, the DI water which has been running to keep the brush moist is turned off and the HF solution is turned on. The substrate is scrubbed for a period of time (which is selected by the user) to etch the substrate such that the desired amount of surface material is removed. Once the etch is complete the HF solution is turned off and the DI water is turned back on. The DI water is turned on after scrubbing with the HF solution in order to rinse the substrate (and the brush) to remove any extraneous HF that may be on the substrate before the substrate is transported out of the second brush station and into another process station. Such a process creates a chemical profile as illustrated in FIG. 2b.
As illustrated in FIG. 2b, the chemical profile on the substrate and brushes is uneven and takes a considerable amount of time before the desired amount of HF is delivered to the substrate. At time 240, the substrate has been placed into the second brush station, the HF solution is turned on, and the DI water is turned off. The HF is dispensed from time 240 to 250. At time 250, the HF dispense ends and the rinse process begins by turning on the DI water and turning off the HF solution.
FIG. 2b also illustrates the amount of time necessary to run such a process. The HF etching process is long because it takes a significant amount of time to reach the desired level of HF and it also takes a significant amount of time to rinse the substrate before the substrate can be transported to the next process station for the next process (e.g., the rinse, spin, and dry station). It should be noted that depending upon the amount of material that the user desires to be removed (or etched), the process time will vary. In other words, the greater the amount of material that is to be etched from the surface of the substrate the longer the etch process time (i.e. the longer the time from 240-250).
It should be noted that, increasing the concentration of the HF may also increase the amount of material that is etched. However, increasing the concentration of the HF solution decreases the control over the etch process due to the highly reactive nature of HF. Thus, due to the potential of damaging the substrate it is not desirable to increase the concentration of the HF solution.
Additionally, as with the NH.sub.4 OH solution ramp up, the sequence for the HF solution ramp up tends to require large amounts of the HF solution. HF solution is an expensive chemical and is also a potentially harmful chemical. Thus, it would be desirous to use less of such costly and potentially harmful chemicals.
At time 260 the substrate is then transferred out of the second brush station and into the next process station. However, as can be seen in the chemical profile of FIG. 2b, the substrate will still have some HF solution on its surface. Transferring the substrate into the next process station will potentially expose that process station to the highly reactive HF solution. The HF solution could potentially damage the next process station due to its corrosive nature and high reactivity. However, the only other alternative to transporting the substrate while some HF solution remains on the substrate is to increase the rinse time in the second brush station. Increasing the rinse time in the second brush station has drawbacks in that either the etch time has to be reduced or the total process time used in the second brush station must be increased in order to account for the longer rinse. Reducing the etch time reduces the amount of material that can be etched and leads to poor metal contamination performance on the substrate. Increasing the total process time of the second brush station is also not desirable since such an increase increases the total process time of the entire cleaning process and reduces throughput of the system.
Thus, what is needed is a method and apparatus, for cleaning and etching substrates that may be used in existing scrubbers, that apply the solutions in a manner that increases the throughput of the system, but that does not use large amounts of chemicals.